Quick answer: 15 mm and 22 mm copper pipes are named by their outside diameter. With typical walls, 15 mm pipe has a bore of about 13.6 mm and 22 mm pipe about 20.2 mm, so 22 mm carries roughly 2.2 times as much water at the same speed. Use 15 mm for individual taps and radiators, 22 mm for main runs.
Almost every UK home has both sizes, and choosing between them is mostly about how much water a pipe has to deliver and how far. This explainer compares the two with real dimensions and simple arithmetic, and sits within our wider plumbing and hardware sizes section. If you are not yet sure which size you have, start with the guide on how to measure pipe size.
What is the difference between 15mm and 22mm copper pipe?
The difference is 7 mm in outside diameter but more than double the internal area. Copper tube in the UK is made to BS EN 1057, and the walls most often used for domestic water and heating are 0.7 mm for 15 mm pipe and 0.9 mm for 22 mm pipe. The figures below follow from those dimensions.
| Property | 15 mm copper | 22 mm copper |
|---|---|---|
| Outside diameter | 15 mm (about 0.59 in) | 22 mm (about 0.87 in) |
| Typical wall thickness | 0.7 mm | 0.9 mm |
| Bore (inside diameter) | 13.6 mm | 20.2 mm |
| Internal area | 145 mm² | 320 mm² |
| Water held per metre | 0.145 litres | 0.320 litres |
| Older imperial equivalent | ½ in | ¾ in (needs an adaptor) |
| Typical household use | Branches to taps, WCs, radiators | Main runs, boiler flow and return, bath feeds |
Internal area is π × (bore ÷ 2)². For 15 mm pipe that is π × 6.8² ≈ 145 mm², and for 22 mm it is π × 10.1² ≈ 320 mm². Divide one by the other and 22 mm pipe has 2.2 times the area.
How much more water does 22mm pipe carry?
At the same flow speed, 22 mm pipe carries about 2.2 times as much water as 15 mm pipe. As an illustration, at a flow speed of 1.5 metres per second a 15 mm pipe delivers about 13 litres per minute and a 22 mm pipe about 29 litres per minute. Those are calculated figures, not ratings: real flow depends on your supply pressure, the length of the run and the number of bends and fittings.
Length matters because friction steals pressure along every metre of pipe, and it does so faster in a narrow bore. That is why a long run to a far bathroom, or a low-pressure gravity system fed from a loft tank, benefits more from 22 mm than a short branch on a high-pressure mains supply does.
When should you use 15mm pipe?
Use 15 mm for short branches that feed a single outlet or appliance. It is cheaper, easier to bend by hand with a spring or bender, and holds less standing water.
- Basin and kitchen tap feeds
- WC cistern supplies
- Washing machine and dishwasher valves
- Radiator flow and return tails, teed off larger heating pipes (see our radiator measuring guide for valve spacing)
- Outside taps
When should you use 22mm pipe?
Use 22 mm where a pipe feeds several outlets, has to deliver a high flow, or runs a long way. Typical places include:
- The main cold supply run after the stop tap in many houses
- Feeds from a loft cold water tank to a hot water cylinder and to bath taps on gravity systems
- Central heating flow and return pipes from the boiler, which then branch to 15 mm at each radiator
- Supplies to high-flow outlets such as bath fillers, where a faster fill is wanted
On many combi boilers, the heating flow and return connections are 22 mm and the hot and cold water connections are 15 mm, but the installation manual gives the exact sizes for your model. Gas supply pipes are often copper too, and their size is calculated by a Gas Safe registered engineer from the appliance’s gas rate and the length of the run. Never alter gas pipework yourself.
How much cold water sits in the pipe before hot arrives?
A 10 m run of 22 mm pipe holds about 3.2 litres, compared with about 1.45 litres for 15 mm. When you open a hot tap, the cooled water in the pipe has to run off before hot water reaches you, so oversizing a long hot-water branch wastes water and time. This is one reason plumbers usually step down to 15 mm for the final run to a tap.
| Length of run | Water in 15 mm pipe | Water in 22 mm pipe |
|---|---|---|
| 1 m (3 ft 3 in) | 0.15 L | 0.32 L |
| 5 m (16 ft 5 in) | 0.73 L | 1.60 L |
| 10 m (32 ft 10 in) | 1.45 L | 3.20 L |
| 15 m (49 ft 3 in) | 2.18 L | 4.81 L |
The same figures help when draining down a system or adding inhibitor to a heating circuit. Convert to pints or gallons with the volume converter if needed.
Can you connect 15mm pipe to 22mm pipe?
Yes, with a reducing fitting. A 22 × 15 mm reducing coupler joins the two in a straight line, and a 22 × 22 × 15 mm reducing tee takes a 15 mm branch off a 22 mm run, which is how most radiator and tap branches are made. Reducers come in compression, soldered and push-fit versions. Teeing 15 mm off 22 mm is normal practice; feeding a 22 mm run from a 15 mm pipe simply limits the larger pipe to what the smaller one can deliver.
Common misunderstandings
- “22 mm gives more pressure.” A bigger pipe does not raise pressure. It loses less pressure along the way, which can improve flow at the outlet.
- “The size is the hole through the middle.” UK copper sizes are outside diameters. The bore is smaller.
- “Bigger is always better.” Oversized hot water branches hold more cold water and take longer to run hot.
- “Old ¾ inch pipe is 22 mm.” It is slightly larger and normally needs an adaptor to join metric fittings.
Frequently asked questions
Is 22mm pipe better for showers?
It can be, especially on long runs or low-pressure gravity systems, because 22 mm loses less pressure than 15 mm. On a short run from a high-pressure supply, 15 mm is often enough. Check the shower maker’s minimum pressure and flow requirements.
What is the internal diameter of 22mm copper pipe?
With the common 0.9 mm wall, 22 mm copper pipe has an internal diameter of about 20.2 mm. 15 mm pipe with a 0.7 mm wall has a bore of about 13.6 mm.
How many radiators can 15mm pipe feed?
There is no single number, because it depends on the heat output of the radiators, the pump and the system design. Radiator tails are normally 15 mm, branched from 22 mm flow and return pipes, and a heating engineer sizes the system from a heat loss calculation.
How much water is in 1 metre of 15mm pipe?
About 0.145 litres, or 145 ml, with a 0.7 mm wall. One metre of 22 mm pipe holds about 0.32 litres.
Do 15mm plastic and copper pipes use the same fittings?
Yes. Both are 15 mm outside diameter, so push-fit and many compression fittings suit either, but plastic pipe needs a pipe insert in most fittings. Check the fitting maker’s instructions.
Sources
- EN 1057 copper tubes: dimensions and wall thicknesses, Engineering ToolBox
- The Water Supply (Water Fittings) Regulations 1999, legislation.gov.uk
- British Standards Institution, publisher of BS EN 1057
- Gas Safe Register